Why are transformers rated in KVA?

Why are transformers rated in KVA? , When Transformer designed that time manufactured company don’t know about secondary side How much will be load power factor of installed load.this reason why transformer rated in KVA.

Second Point is , Transformer is power transformer device not power consuming device , so it unit will in KVA (Kilo Volt Ampere) not a KW.(Kilowatt )

Genarator unit also in kva cause its power transformer device not power consuming divice.